One of the primary general performance goals for a bar screen apparatus is to combine a high capacity for receiving and sorting wood chips with a low accepts carryover rate, i.e., a low rate of chips of acceptable size for the particular bar screen proceeding off the outfeed end of the bar screen. This goal is applicable for all bar screen apparatus, regardless of the particular configuration and/or arrangement of the bar screen itself. For bar screens which are designed to process the wood chips in a thickness dimension, a chip loading rate in the range 0.30-0.50 units per hour per square foot of screen surface (UPHPSF) is typical. Within this loading range, with an overthick removal efficiency (ORE) of about 85% or greater, an accepts carryover rate (ACR) in the range of 5-20% is normally acceptable.
A bar screen which significantly exceeds the above performance would be very competitive in the industry, while a bar screen which performed below such a standard would be at a competitive disadvantage and likely would not be well accepted in the industry.
Various attempts have been made to improve bar screen performance by varying one or more of the following parameters: the direction and/or amount of inclination of the bars, the speed of the bars, and the configuration of the bars. These attempts, directed generally toward maximizing capacity while minimizing accepts carryover, while maintaining overthick removal at an acceptable level, have met with varying success.
One recent example is shown in U.S. patent application Ser. No. 07/918,645, in the name of Gevan R. Marrs and assigned to the same assignee as the present invention. That application is directed toward a particular kind of bar screen using thin, blade-like bar elements on the order of 1/16 inch thick, instead of the relatively thick, rigid (approximately 1/2 inch thick) bars used in conventional bar screen systems. This "blade screen" arrangement, in which the bars are held in tension at opposing ends, has significant performance promise because the percentage of open area of the screen is quite high, within the range of 50-90%, compared to a much lower percentage of open area, usually around 35%, for bar screens using conventional bars. However, adequate loading capacities have been difficult to achieve with the blade screen arrangement.
In addition, bar screens using more conventional bars, with lower percentage of open area figures, would also benefit significantly in performance if loading capacity could be increased without affecting the accepts carryover rate.
Accordingly, it is desirable to increase the capacity of bar screens while maintaining a high overthick removal and an accepts carryover rate within an acceptable range, in order to improve overall performance. This is particularly desirable for bar screens using relatively thin bars; i.e. those bar screens having a large percentage of open area, which appear to possess a structure capable of achieving relative high loading capacity.